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Food Microbiology

Background & Introduction: 

Perhaps when we think about microorganisms and food, we are most likely to visualize the role that these microorganisms play in food spoilage. Species of Pseudomonas, Erwinia, and many of the lactic acid bacteria are common spoilage bacteria. While these bacteria lead to distasteful and often smelly food, they are generally nonpathogenic. However, if food is contaminated with a large number of spoilage organisms, this can indicate the presence of foodborne pathogens, thus spoiled foods should not be consumed. 

There are two types of foodborne illness. The first, food intoxication, results from the ingestion of a pathogen-produced exotoxin that contaminates food. Staphylococcus aureus produces an exotoxin that is an enterotoxin. It causes nausea and vomiting. Although S. aureus generally does not compete well with most food spoilage organisms, it can thrive on salty foods that are left out at room temperature. Clostridium botulinum is another bacterium that produces an exotoxin. This bacterium is anaerobic and thus can thrive in canned foods that are damaged or that were improperly heated. The exotoxin produced by this bacterium is a neurotoxin that can cause the paralytic disease, botulism. 

The second type of foodborne illness is a food infection. This type of disease requires the actual consumption of the microorganism itself. The ingested bacteria often grow and invade the epithelial intestinal lining. This invasion commonly followed by the secretion of toxins leads to diarrhea, fever, vomiting etc… Food infections are caused by a variety of bacteria such as Campylobacter, Salmonella and Escherichia coli 0157:H7 

In contrast to their role in spoilage and disease, microorganisms play a large part in food production and preservation. Microorganisms are essential in the production of wine and beer. Cheese, yogurt, buttermilk, sour cream and other fermented milk products are produced by the lactic acid bacteria. The aptly named, Acetobacter is used in the production of vinegar. The acid byproducts that flavor these foods also serve to preserve them. The lactic acid bacteria (LAB), in addition to their acidic byproducts, also produce small antimicrobial peptides called bacteriocins. These bacteriocins help the LAB compete in the food medium on which they grow. Some of these bacteriocins are currently being used in the food industry to help in preservation. 

The LAB bacteria are not alone in their ability to produce antimicrobial agents. Many larger organisms produce these agents and they are thus found naturally in a variety of food products that we consume. Milk, for instance, contains a compound called lactoferrin, which sequesters iron. It also contains antibodies and an enzyme called lactoperoxidase that enacts oxidative damage. Eggs also have antibodies and the enzyme lysozyme, which targets peptidoglycan. Some of the strongest natural antimicrobials are found in food products of plants. Spices such as cloves, allspice, oregano, rosemary, sage and vanilla contain phenolics that denature bacterial proteins and perturb membranes. Garlic contains a variety of antimicrobials, one of which, allicin, is thought to inhibit bacterial metabolism. Green tea has compounds called the catechins and while the mechanism of action of these compounds is unclear, they are known to have an antimicrobial effect. [Background information for the above discussion was obtained from Nester et. al. (ch. 32), lecture notes by Dr. Rick Holley (University of Manitoba), BMC Microbiol. 2005 Jun 17;5(1):36. Microbes Infect. 1999 Feb;1(2):125-9] 

In this lab procedure, E. coli, S. aureus and P. aeruginosa will be tested for their susceptibility to a variety of natural antimicrobials found in food. 


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